Method of separating neptunium from plutonium in aqueous inorganic solution



United States Patent FROM SOLU- This invention relates to a process for separating neptunium and plutonium.

When uranium compositions are subjected to the action of neutrons as in a neutronic reactor, U captures a Calif., assignor to the United these'acids mayia son er eia neutron and forms U This is a short-lived isotope of uranium which decays promptly to Np This isotope of neptunium is a short-lived beta emitter w ch decays to plutonium, Pu

When such neutron bombarded uranium is allowed to age after bombardment for a number of months, most of the neptunium shall have decayed and plutonium essentially free of neptunium is present. However, when only a short period of cooling or decay is used, neptunium will be present. In addition other isotopes of neptunium will be found which have longer half lives than does Np It is an object of this invention to provide a composition comprising neptunium essentially free of plutonium.

A further object of this invention is to provide a means of separating neptunium from plutonium.

In accordance with this invention neptunium and plutonium are separated by contacting a solution containing these elements in their reduced states with an oxidizing agent having a standard oxidation-reduction potential substantially more negative than minus one volt and having a negligible oxidation rate for plutonium and a substantial oxidation rate for neptunium, adding a carrier that will carry from solution these elements only when present in an oxidation state not greater than four, and thereafter separating It has been found that plutonium is chemically unlike osmium in many respects, and is probably a member of a second rare earth group, the actinide series. It has further been discovered that plutonium, unlike a number of other members of this series, possesses a plurality of valence states. Plutonium has at least four valence states, including +3, +4, +5, and +6. In 0.5 M1.0 M aqueous hydrochloric acid the oxidation-reduction potentials are of the following magnitudes:

Volts Pu+ Pu+ +E-' -0.97 Pu+ +2H O- PuO ++4H++E- -1.11 PuO PuO +E- -0.92 Pu "*+2l-I O lPuO +4H++213.- -1.0l5

As may be seen from the above couples, the stability of the higher oxidation states is dependent on the hydrogen ion concentration. In moderately acidic solutions the Pu ion is generally very unstable, and disproportionates to Pu and Pu+ The Pu+ ion is capable of disproportionating to the Pu+ ion and the PuO ion, and in dilute aqueous hydrochloric acid this disproportionation may take place to a considerable extent. The Pu+ disproportionation is opposed, however, by increase in hydrogen ion concentration and by the presence of ions which tend to complex or otherwise stabilize the Pu+ ion. The eifect of additional ions in hydrochloric acid solutions is illustrated by the following potentials for the Pu+ Pu+ couple:

Volts 1.0 M HCl O.97 1.0 M HCl-0.l M H PO -().80 1.0 M HCl-l.0 M HF 0.53

the precipitate from the supernatant.

ice

Generally the anions of slightly ionized acids tend to complex the Pu+ ion to a much greater extent than the anions of highly ionized acids. Thus, Pu+ is only slightly complexed by ClO Cl, and N0 1 it is complexed to a much greater extent by 50 and it is very strongly complexed by P0,, F, C H O and C 05 In addition to the complexing eilect of the anions of the acids employed as solvents for plutonium, certain of oxidizing agents. However, moderately elevated tempera= of oxidation catalysts, the rate of oxidation by the acid is often so low that this effect may be ignored. Thus, the PM" ion is stable for considerable periods of time in perchloric acid, although under proper conditions, the latter is capable of oxidizing Pu+ to PuO It is therefore desirable to control the state of oxidation of the plutonium by the use of oxidizing agents and reducing agents which have rapid reaction rates under the conditions employed for processing the solutions.

The Pu+ ion may suitably be oxidized to the PuO ion by the addition of an active oxidizing agent having an oxidation-reduction potential substantially more negative than the oxidation-reduction potential of the Pu+" PuO +2 couple in the particular solution employed. The following are representative potentials for this couple:

at room temperatures, or tures, and in the absence Volts 1.0M HCl -1.0 1.0 M HNO -1.1 1.0 M H 1.3

Oxidizing agents having adequate oxidation-reduction potentials for use in such solutions may be chosen by reference to tables such as the table of standard oxidation-reduction potentials given in the Reference Book of Inorganic Chemistry by Latimer and Hildebrand (The MacMillan Company, New York, 1934).

It is generally desirable to eflect purification and concentration of plutonium in nitric acid solutions. Examples of oxidizing agents for use in such solutions are bromates, permanganates, dichromates, silver-catalyzed peroxydisulfates, and ceric compounds. To efiect the oxidation, a quan -ty of oxidizing agent at least equivalent to the amount of plutonium is added to the solution, and the resulting mixture is digested at a moderately elevated temperature for a suflicient period of time to insure complete oxidation of the plutonium. In most cases, this digestion may suitably be effected at 60-80 C. for 15-60 minutes. In order to maintain the plutonimum in the hexavalent state for considerable periods of time after oxidation, it is desirable to employ an excess of oxidizing agent to serve as a holding oxidant. This is especially true if an acid solution is to be processed in ferrous metal equipment, or under other conditions permitting subsequent reduction of the plutonium.

Neptunium may be oxidized by any of the oxidizing agents mentioned above, without the necessity of digestion at an elevated temperature. This greater rapidity of oxidation of neptunium at low temperatures may be utilized to effect preferential oxidation of neptunium without substantial oxidation of plutonium. The preferred oxidizing agent for this purpose is the bromate ion. At temperatures of 15-25 C. neptunium may be substantially completely oxidized by alkali metal bromates in nitric acid solutions, which contain ions such as 80.;- ions to complex +4 plutonium, without appreciable oxidation of plutonium to the hexavalent state. There is some evidence that bromate oxidation of plutonium may be catalyzed by cerium, and it is therefore desirable to effect the preferential oxidation of neptunium in cerium-free solutions.

7 standard potentials such as the table previously referred to.

The reduction may suitably be effected by digestion at employing reducing agents having oxidation-reduction potentials of the same magnitude as hydrogen peroxide and ferrous iron. However, it is also possible to use stronger reducing agents such assulfur dioxide if any exroom temperature or slightly elevated temperatures. 5 cess reducing agent is removed or destroyed after the Digestion for 15 to 60 minutes at 15 to 35 C. will geninitial reduction'is eifected. In any case, if Pu+ is deerally be satisfactory. sired, the hydrogen ion concentration should be sufiicient- For the reduction of PuO or Pu+ to Pu+ the reducly high to oppose the disproportionation of Pu+ to Pu+ ing agent should have an oxidation-reduction potential and PuO For this purpose, it is desirable to employ substantially more positive than the oxidation-reduction solutions having a pH not substantially above 2, and potential of the Pu+ -+Pu+ couple in the solution em preferably considerably below 1. In the case of aqueous ployed. Thus, in 1.0 M HCl an active reducing agent nitric'acid solution, it is generally desirable to maintain having a potential more positive than 0.97 v. will be re a free acid concentration of at least 1 M. quired, and in 1.0 M HNO a potential more positive than It will be apparent that the considerations discussed -0.92 v. will be necessary. In order to maintain the pluabove will also apply to the oxidation of Pu+ to P ntonium in the +3 valence state for appreciable periods of without oxidizing Pu+ to PuO+ by the use of oxidizing time, it is desirable to maintain an excess of the reducing agents having potentials intermediate the potentials of agent in solution. the two plutonium couples.

In order to reduce PuO to Pu+ without reducing The plutonium oxidation and reduction processes de- Pu+ to Put it is desirable to employ an active reducing scribed above may be employed, if desired, for the simulagent having an oxidation-reduction potential substan taneous Oxidation or reduction of both neptuniumand tially more positive than the oxidation-reduction potential plutonium. Such simultaneous oxidation or reduction of the Pu+ PuO couple, and substantially more negaill be attained provided equilibrium is reached. As pretiv than th o id ti n-r d ti potential of th viously pointed out, however, differential reaction rates Pu+ Pu+ couple, in the solution used. A wider selecmay be utilized to attain one oxidation state for neptunium tion of reducing agents of the desired potential will be nd another oxidation state for plutonium. available for use in solutions containing ions which com- The s l ti s of p utonium ions of the various valence plex the Pu+ ion than are available for use in solutions states described above are useful for the electro-deposition which are substantially free from complexing effects. of Plntnnl'unl, for the Precipitation of Plutonium Thus, in 1.0 M'HCl and 1.0 M HCl-1.0 M HF, the oxipounds while leaving contaminating compounds in soludation-reduction potentials are approximately: tion, and for the precipitation of contaminating pounds while leaving plutonium in solution, as will be LO M H01 m M discussed in detail in the description of other phases of 1.0 M HF the present invention.

The oxidation state of plutonium in aqueous solutions V. V. of the various plutonium cations may be determined in 33i;3gfigfi j j :5 2, ,1? accordance with methods commonly used for the deter- V mination of the valence state of other metals in solution.

It may be seen that in the solution containing fluoride Thus toial P solumPn Y be detefmmed ion Ieducing agents such as hydrogen peroxide and by quantitative gravimetric or radlometrrc analysis, and rous iron, which have oxidation-reduction potentials of pfircentage. of y partlqular Ion then be l -0.68 v. and 0.74 v. respectively, will reduce Puo gamed i a.sultable dlfierfentlal analysis 5 as quaPtlta' only to Pu+ whereas in the solution without fluoride ion tlve pxldatlon or reductlon. T 3 anaiysls or to complex the Pu+ ion, these reducing agents will tend the spectrophoioinemc a l 'f x to reduce the plutonium to the +3 state. A reducing Vantage? for detefmmfng qflahtatlvfaly f qufmtltatlvely agent such as sulfur dioxide, having an oxidation-reduction the Vanous plufofnum 111 solutlon, 111 View of the potential of O.14 v. will tend to reduce the plutonium to sharp charactenstlc Peaks 1n the absorption Spectra for h +3 State i i h l i the diflerent valence states. Representative molar extinc- When employing the preferred solutions of plutonium n Coeificients for the and Q2 inns in in aqueous nitric acid, the reduction of PuO to Pu+ aqueous inorganic acid solutions are given in the followis preferably effected in the presence of a complexing ion, ing tables:

TABLE 1 [Pu+ 1n 1 M H01] Wave Lengthin .4 4, 260 4,560 4,740 5,050 I 5,620 6,010 6, 660 8,000 I 9,090 Molar Extinction Coefficient--- 12.0 4.7 4.0 3.5 37.4 37.9 15.0 15.0 18.9

TABLE 2 Wave Lengthin A..." 4,040 4,220 4, 480 4,760 5, 020 5, 460 6,600 7,080 8,000 8, 550 Molar Extinction O0- efiicient 27.0 24.5 17.5 72.5 8.7 17.0 31.0 14.0 18.9 13.2

TABLE 3 [Pu' 1n 1 M His 0d Wave Length in A 4, 090 I 4, 360 4,810 5, 480 I 6,640 I 7,200 I 8,140 8,510 Molar Extinction Coefiilcient 29.2 28.5 85.2 20.0 39.6 21.0 27.1 14.3

TABLE 4 [PuOi in 1 M HNOJ] Wave Length in A 4, 590 4, 700 5,060 Molar Extinction Coefficient 15.0 14. 14.0

A method for separating plutonium and neptunium from aqueous solutions containing uranium, neptunium, plutonium, and fission products is disclosed in copending application Serial Number 637,485, filed December 27, 1945, of which the present invention is a continuation-inpart. According to that invention, plutonium and neptunium are effectively separated from uranium and fission products by contacting a solution containing said elements with lanthanum ions and fluoride ions, thereby precipitating lanthanum fluoride which carries plutonium and neptunium also as fluorides. In this phase of the process neptunium and plutonium are in a valent state not greater than plus four. A separation of plutonium and neptunium from fission products of the rare earth type is effected by oxidizing the plutonium and neptunium to a valent state above plus four, adding lanthanum ions and fluoride ions which form a lanthanum fluoride precipitate thus carrying rare earth elements from solution, and then separating the supernatant solution, which contains substantially all the neptunium and plutonium from the precipitate.

The instant invention provides a means of separating neptunium from plutonium by oxidizing the neptuninmarl l not the plutonium and then carrying out a lanthanum fluoride precipitation step thereby carrying down the plutonium and leaving the neptunium in solution.

Carriers other than lanthanum fluoride may be used in separating nepeanium iromzplnmmumnbyisuch a procedure. Thus, bismuth phosphate may be used to carry the plutonium from a solution containing plutonium in a valent state not greater than plus four and neptunium in a valent state greater than plus four.

The use of a carrier precipitate to separate plutonium from an aqueous solution, leaving neptunium in a higher oxidation state in the supernatant liquid, is illustrated by the following example:

Example 1 A 4.3 M sulfuric acid solution was prepared, containing lanthanum sulfate in a concentration of approximately 430 mg. per liter and plutonium and neptunium in tracer concentrations. To this solution was added about 2.1 times its volume of an aqueous solution 0.2 M with re spect to bromate ion and 0.2 M with respect to bromine. The resulting solution, which had a lanthanum sulfate ing the resul-ti concentration of about 1 39 mg. per liter, and was about 1.4 M with respect to sulfuric acid, about 0.14 M with respect to bromate ion, and about 0.14 M with respect to 10 bromine, was allowed to stand at room two hours to effect oxidation of the neptunium to the hexavalent state while leaving the plutonium in the tetravalent state. About 27% temperature for by volume of 48% aqueous acid was then added to the solution, and

the resulting lanthanum fluoride precipitate was separated by centrifuging. Analyses for alpha and beta radiation showed that the precipitate contained 99% of the plutonium which was present in only 0.74% of the neptunium.

What is claimed is: 1. A process for the separation of plutonium in an aqueous neptunium, plutonium and sulphate ions,

the original solution, but

neptunium from inorganic solution containing comprising contacting said solution with an alkali metal bromate, digest- C. for a period of time not more than oxidize substantially num ions and fluoride ions,

30 natant solution.

with respect to bromate ion 2. A process for ng mixture at a temperature of 15 to 25 that required to all of the neptunium, adding lanthaand separating the plutonium-containing precipitate thus formed from the superthe separation of neptunium from plutonium in an aqueous inorganic solution containing neptunium, plutonium and sulfate ions, comprising incorporating in said solution an aqueous bromine, digesting the resulting mixture solution 0.2 M

and 0.2 M with respect to at a temperature of 15 to 25 C. for a period of time not more than that required to oxidize substantially area-see aeptanium adding lanthanum ions the supernatant solution.

and fluoride ions, and separating 40 the plutonium-containing precipitate thus formed from kefereaees iteain .ihenfileoflh icl at UNITED STATES PATENTS Thompson et a1 Thompson et al. Seaborg Mar. 19, 1957 July 16, 1957 Oct. 29, 1957 OTHER REFERENCES Seaborg and Wahl: Journal of the American Chemical Society, vol. 70, pp. 1128-1134 (1948) Strassmann et al.: The Isolation and Some of the Propenschaften, vol. 30, pp.

The Transuranium Elements, part pp. 25-38. Na- IV, volume 14B 

1. A PROCESS FOR THE SEPARATION OF NEPTUNIUM FROM PLUTONIUM IN AN AQUEOUS INORGANIC SOLUTION CONTAINING NEPTUNIUM, PLUTONIUM AND SULPHATE IONS, COMPRISING CONTACTING SAID SOLUTION WITH AN ALKALI METAL BROMATE, DIGESTING THE RESULTING MIXTURE AT A TEMPERATURE OF 15 TO 25* C. FOR A PERIOD OF TIME NOT MORE THAN THAT REQUIRED TO OXIDIZE SUBSTANTIALLY ALL OF THE NEPTUNIUM, ADDING LANTHANUM IONS AND FLUORIDE IONS, AND SEPARATING THE PLUTONIUM-CONTAINING PRECIPITATE THUS FORMED FROM THE SUPERNATANT SOLUTION. 